Why Urine Tests Are the Least Reliable DUI Chemical Test

If given the choice, a driver should choose a urine test over other chemical tests

If you’re in the unfortunate position of knowing you had way too much to drink and are offered a choice to take a urine test, you should choose it over other tests. That’s because of the three common tests -- blood testing, breath testing and urine testing --  most DUI experts agree that urine testing is the least accurate and therefore the most likely to be deemed inadmissible. Because of that, urine testing has been disfavored in most states and is given only as a test of last resort – that is, when the other chemical tests are not available. Urine testing is still comon for drug testing -- versus alcohol testing -- even though the presence of a drug in urine does not always provide reliable information as to whether the driver is actually under the influence at a specific time. Many experts also believe that the concentration of drugs in urine is not reliably correlated to blood concentration as discussed with alcohol, below. 

Why urine testing is disfavored

Correlating the numbers. One reason why urine testing is often inaccurate is that the percent of alcohol in the urine is not necessarily the same as in a person’s blood. Experts agree that the level of alcohol in urine is typically about 1.33 times the BAC level. So, to convert a urine test result into an equivalent blood alcohol level, it’s necessary to divide the urine alcohol level by 1.33. However, this number is an average, and a driver accused of a DUI can argue at trial that this average figure didn’t apply to the particular test at hand. For example, some studies have shown that some people have alcohol levels only 40% as high in their urine as in their blood, while others have twice the alcohol content in their urine as in their blood. By the way, a urine sample will normally be preserved so you can also arrange for an independent test. 

How the bladder functions. Another reason why urine testing is disfavored is the way in which the bladder functions. Because urine is stored in the bladder and stays there until the bladder is emptied, the contents of the bladder represent a shifting composite of a continuously changing blood alcohol content. In this way, the bladder’s contents tell much less about a person's blood alcohol at a particular moment than does a blood sample. This fact of nature can work for or against the driver charged with a DUI.

  • Example 1: Drinking hours ago. Let’s say that four hours earlier, the driver had four boilermakers and hasn't urinated since downing his drinks. His urine test results could be misleadingly high. That is, his bladder may have a high disproportionate amount of alcohol compared to his BAC.
  • Example 2: Drinking recently. Alternately, let’s say that an hour or so before being tested, the driver had a jug of Gatorade, and then 15 minutes before being stopped had  a few shots of Johnny Walker. Because all of alcohol may not have reached the bladder, and because of the diluting presence of the Gatorade already present, the urine test would give a misleadingly low result. For this reason, the police sometimes insist that a driver produce a second sample 20 minutes after the first. If that's impossible, the driver will have to take the more accurate blood test or the breath test.

Same old problems. Finally, urine samples are analyzed for alcohol in almost the same way as blood samples. The results are therefore also subject to some of the same laboratory errors.

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